The invention relates to the transmission of digital data over a network comprising a data transmission line and a synchronization and control line connecting a master station alone capable of transmitting over the control line and several slave stations.
It finds an important, although not exclusive application in data acquisition systems in which the slave stations comprise a concentrator of the data coming from sensors associated with the station, as well as digitizing means if some at least of these sensors are of the analog type.
Numerous data transmission networks are already known for transmitting data, organized either in a bus line (i.e. not looped) or in a looped ring. Among the first may be mentioned the Hi Net master-slave network with bus and transmission over a twisted pair. Among the second may be mentioned the "Prime Net" token passing network, with ring topology and base band transmission over coaxial cable.
Transmission over these networks is generally asynchronous. But, because of the absence of phase relationship between the stations, each transmission of useful information must be preceded by a sync data exchange which considerably reduces the capacity of the network. In the frequent case where it is desired to reach high transfer rates, synchronous transmission is much more advantageous. But it requires having available at reception clock signals synchronized with those of the emission. A solution frequently used consists in extracting the clock signal from the digital data train received and using it for decoding: but this technique generally involves the use of phase lock loop type devices which require a synchronizing sequence preceding said message on which the latching may operate.
The length of this synchronizing sequence increases with the transmission frequency and may become considerable compared with the size of the smallest passage transmitted. Another solution consists in transmitting the clock signal used for transmitting the message over a separate channel, so as to allow immediate decoding. This technique requires the use of an additional channel to which all stations have access for transmission and reception.
A digital data transmission system is moreover known (FR-A-2,254,253) comprising both a transmission line and a control line. But there is no continuity of transmission over the control line, in a form allowing permanent extraction of the clock signal by the slave stations, and so absolute suppression of the risk of synchronization loss. More precisely, the document FR-A-2,254,253 describes a network whose "desks", i.e. the slave stations able to transmit, must be disposed in a hierarchical order in the loop (page 5, lines to 18-21) and communication with "chambers" takes place in sequence, by counting clock pulses. The sync line has only flowing therethrough clock signal bursts separated by breaks. A network is also known (US-A-3,601,543) with time slots assigned to different stations. The data signals applied to the data line are formed by modulating the sync signals received over the sync line. For that purpose, a means coupling the sync line and the data line is selectively enabled and disabled. Only successive sync frames, each preceded by a start signal, are transmitted over the sync line.
It is an object of the invention to improve upon the prior art methods of synchronous transmission of digital data over a network; it is a more specific object to ensure permanent synchronism of the clocks of all stations in a simple way.
To this end, a process according to the invention for transmitting digital data over a network comprising a data transmission line and a sync and control line connecting a master station, alone capable of transmitting over the control line, and several slave stations, includes the steps of circulating the data in phase over the two lines which have the same configuration and maintaining continuous tranmission over the control line by sending over the control line from the master station characters suitable for extraction of sync signals between the control data; at each slave station, the clock signal is continuously extracted from the information flowing over the control line.
In a preferred embodiment, the lines are unidirectional and form two parallel loops closing again on the master station.
The code used on the control line is chosen from those allowing the clock to be reconstituted from the coded message (Manchester code for example). Clock extraction is facilitated by the continuity of the transmission of characters over the control line, which may be provided by a simple device placed at the master station. Thus, there is no further need of the delay separating two successive transmissions by the master station on the control line nor of a synchronizing sequence preceding the messages transmitted over this line.
The invention also provides a data transmission installation comprising a data transmission line and a sync and control line connecting together a master station, alone capable of transmitting over the control line and several slave stations, characterized in that the two lines are unidirectional and looped back to the master station and in that the stations transmit information to phase over the two lines, each slave station comprising means for extracting clock information from the signal on the control line. Since the clock is extracted at each slave station from the information flowing over the control line, which information is in phase with the data flowing over the data line, the information flowing over the data line may be decoded, transmitted and, possibly, repeated in a very simple way; thus, the need of having any synchronizing device in the data line and any synchronizing pattern in the data transmitted is removed. Because of the monodirectional character of the two lines, under the control of a single clock, the data flows in the same direction and with the same travel times over the two transmission lines, which maintains the clocks in phase with the binary trains received.
The physical transmission medium and the method of modulation may be of any kind, provided that they are compatible with the pass-band required. If the medium is non diffusing, there may be physical continuity of the transmission medium from the transmission access of the master station to the reception access of this same station, over the two transmission lines. If, on the other hand, the physical transmission medium forming the data line is diffusing, it must be interrupted at each station and each slave station plays the role, among others, of repeater for the information transiting over the data line.